A growing body of correlative evidence has implicated free radicals as a important factor in the pathology of both Parkinson's disease (PD) and normal brain aging. Dopaminergic nigrostriatal neurons, the predominant cell type lost in PD, are believed to be highly susceptible to free radical damage due to the propensity for dopamine to oxidize, producing elevated levels of reactive oxygen species. This problem is exacerbated by region-specific decreases in antioxidant defenses in the proximity of the substantia nigra. De-regulation of the glutathione system, a major component of the antioxidant defense system in the brain, has been strongly implicated as a causal factor in the resulting neuronal degeneration, but a direct role has yet to be definitively proved. We propose in this study to use genetic engineering to directly assess the involvement of the glutathione system in protecting the brain against neuronal damage due to either acute and chronic oxidative stress, and to test whether deregulation of this system can result in neurodegeneration like that seen in PD or during the normal aging process. This will be done using tissue culture and animal models in which levels of glutathione are altered by either over- or underexpression of glutamylcysteine synthase (GCS), the rate limiting enzyme in the synthesis of glutathione, or genetic over- or underexpression of glutathione peroxidase, the enzyme which acts with glutathione to de- toxicity reactive oxygen species. Such in vitro and in vivo systems should allow us to explore the hypothesis that genetic variations in levels of these molecules could be involved in predisposition to or protection against Parkinson's or neuronal degeneration during normal aging.